In order to grow such edible mushrooms as Flammulina velutipes, Pleurotus ostreatus, Hypsizigus marmoreus, Pholiota nameko, and Grifola frondosa, it is known to use a culture medium containing such nutrients as sawdust, rice bran and wheat bran. Before inoculating a mushroom spawn to the culture medium, it is put in a plastic bag or a glass container, sterilized for one hour at 120.degree. C. under the pressure of 1.2 atm. in a high-pressure sterilizing oven (autoclave), and then cooled. Now, this technique is also employed in growing Lentinus edodes.
This method is superior to the conventional one in which mushroom spawns are inoculated on logs, because it saves a lot of labor and permits a planned mass-production of mushrooms. Thus, this technique is now widespread.
But in this method in which sawdust or the like is used as a culture medium, it takes a long time to sufficiently mature sawdust for use as nutrients for fungi. Thus the productivity is low and the cost too high.
Further, wood of conifers from which sawdust is produced, contain some growth inhibitors for mycelium such as terpene compared to broad-leaved trees. Further, the speed at which such sawdust is decomposed and the mycelium is matured is rather slow. Its use is thus difficult. Actually, sawdust of conifers such as Cryptomeria japonica is used together with an excessive amount of nutrients as a substitute for broad-leaved trees. But because such a culture medium contains an excessive amount of nutrients, pathogenic germs such as Trichoderma and other various germs such as green mold tend to grow.